1. Field of the Invention
The invention relates to television tuners, and more particularly, to a television tuner having a single phase-locked loop used to process a received RF signal.
2. Description of the Prior Art
One of the most significant costs in television manufacturing is the cost of the tuner. Furthermore, with the increasing desire to integrate TV functions into personal computer (PC) systems and other electronic devices, the cost of the tuner needs to be reduced. TV tuners may be fabricated on circuit boards and then installed in personal computer systems, thereby allowing the PC to function as a television set. These tuners convert a radio frequency television signal into a baseband (or low frequency) video signal, which can then be passed on to other elements in the PC for video processing applications.
FIG. 1 shows an integrated television tuner 100 as disclosed by Rotzoll in U.S. Pat. No. 5,737,035. The television tuner 100 includes an on-chip circuit 102 in addition to an external off-chip band-pass filter 104. The on-chip circuit 102 includes an adjustable low-noise amplifier (LNA) 106, a first mixer 108, a second mixer 110, a second intermediate frequency amplifier 112, a plurality of first voltage controlled oscillators (VCOs) 114, a first phase-locked loop (PLL) 116, a second VCO 118, and a second PLL 120.
The adjustable LNA 106 amplifies a selected channel in a received RF signal (RF_in), and the first mixer 108 mixes the result with a first local oscillator signal LO1 to up-convert the selected channel to a constant frequency at a first intermediate frequency signal IF1, which is typically at 1220 MHz. To perform this task at a plurality of different selected channel frequencies, the first local oscillator signal LO1 must have a wide frequency range to allow different channels at different frequencies to be up-converted. Thus, a plurality of VCOs 114 must be used. Each VCO has a smaller frequency range and therefore a smaller resulting lower phase noise. The first PLL 116 is used to select the appropriate VCO and to control the frequency of the selected VCO to thereby generate the first local oscillator signal LO1.
The resulting first intermediate frequency signal IF1 is filtered by the off-chip band-pass filter 104 then mixed by the second mixer 110 with a second local oscillating signal LO2 to produce an output signal, which is typically a second intermediate frequency signal at 44 MHz. The second PLL 116 is used to control the frequency of the second local oscillator signal LO2, which is typically set to 1176 MHz. As the mixer 110 would also mix an image signal located at 1132 MHz with the second local oscillator signal LO2 (1176 MHz) to produce an output signal at 44 MHz, the first intermediate frequency signal IF1 is first filtered by the off-chip band-pass filter 104 to partly remove the image signal. Furthermore, the second mixer 110 is implemented as an image rejection type mixer to further prevent the image signal from appearing in the output signal IF2 and degrading the performance of the television tuner 100. Additionally, because the output signal IF2 of the television tuner 100 is not at baseband, an additional frequency conversion stage is needed (not shown). This additional frequency conversion stage requires at least one additional mixer, a third VCO, and a third PLL to control the frequency of the third VCO.
The use of the off-chip band-pass filter 104, the special image rejection mixer for the second mixer 110, and the multiple PLLs to control the frequencies of each local oscillator signal significantly increases the cost and the complexity of the television tuner 100. Accordingly, a need exists for a television tuner not having these requirements in order to reduce the cost and simplify the design.